After scores of complaints and lawsuits concerning the physical and mental abuse of immigrants detained in county jails and other detention centers, the Immigration and Naturalization Service has issued national standards for the treatment of its detainees. The new standards, covering everything from visiting policies to grievance procedures, will be phased in this month at all detention centers administered by the immigration service. They will be phased in over the next two years at state and local jails that house immigration service detainees. But critics say the agency has fallen far short of that goal, especially in county jails in states like Louisiana, Texas, New Jersey and Florida, where detainees and their lawyers say prisoners are beaten, solitary confinement is imposed for trivial offenses, and water and food are often inadequate. Advocates for immigrants say conditions may not improve, even after the standards are in place, because they don’t have the weight of law and could prove impossible to enforce.